=============
Overview
=============

---------------
 Functionality
---------------

   - Calculate :math:`\Delta`\SCF-DFT excitation energies by changing band 
     occupation

   - Constrain and occupy an orbital of a subsystem or reference system to resemble an electronic 
     excitation - le :math:`\Delta`\DSCF-DFT

   - Put penalties onto orbitals of a subsystem in a DFT+U fashion (DFT+U(MO)). This can 
     be done by enforcing idempotency (integer occupation) or by constrained DFT

   - Generate a projection of the orbital of a subsystem on the Density-of-States (DOS)

   - A post-processing tool (MolPDOS) generates the corresponding DOS out of 
     the projection information

---------------
Prerequisites
---------------

   - The following applies to all modes, except the :math:`\Delta`\SCF-DFT mode.
     Nonetheless, this runmode requires a good understanding of its limitations.

   - The two systems should not be strongly geometrically or electronically 
     mixed and a separation into 
     general system and subsystem has to still be chemically reasonable. Otherwise 
     this Ansatz breaks down. Ideally the difference can be seen as a weak 
     perturbation. 
     This applies for example for
         
        + A molecule adsorbed at a metal surface with weak to medium hybridization 
           , *e.g.* C\ :sub:`6`\ H\ :sub:`6`\@Au(111).

        +  A particle or molecule inserted into a porous nanostructure. 

        +  The exact same system in another electronic state, *e.g.* 
           groundstate *vs.* first excited state. 
 
   - The wavefunction of the subsystem has to be calculated with 
     **exactly** the same settings (K-points, spin-polarisation, cutoff, cell size)
     as the actual system. This introduces some artificial dispersion 
     for gasphase molecules, but if the cell is sufficiently large 
     the effects should be small. In addition, this might be something desirable, 
     for example when high coverage situations are investigated. 
 
   - At the moment, the excitation constraints only work for *metals_method='DM'*
     and without non-local exchange, as well as spin_treatment='scalar' or 'none'.

------------
General Use
------------

The general use of this module always involves the following steps:

1) Define the subsystem and calculate the self consistent density and wave functions. 
       This has to be supplied to the actual calculation as checkfile.
       (not for class. :math:`\Delta`\SCF-DFT)

2) in <seed>.param issue one of the following, depending on what
       you want to do

::
         
         %BLOCK DEVEL_CODE
           DeltaSCF           # starts a DeltaSCF or DFT+U(MO) calculation
           MolPDOS            # outputs MO projected DOS data
           DeltaSCF MolPDOS   # starts a DeltaSCF calculation and outputs MOPDOS
         %ENDBLOCK DEVEL_CODE

3) Supply a <seed>.deltascf file that contains the necessary 
       information for the DeltaSCF and/or MolPDOS run.

4) Start a singlepoint calculation. Put task: singlepoint in
       the <seed>.param file and issue

::

         castep <seed> 

5) Only for MolPDOS calculations: Run the MolPDOS post-processing tool. 
       This needs a <seed>.molpdos input file. Herefore you issue

::

         MolPDOS <seed>

---------------
Implementation
---------------

This module and the corresponding post-processor are written in FORTRAN90 
and are part of CASTEP. They heavily make use of the excellent infrastructure 
of the code. The portability is therefore strongly limited.